The present invention relates to a double screen or double-rack grate for use in the extra-fine screening of waste waters, and including two rows of rods. A first row of rods is positioned upstream of a second row of rods, relative to a direction of flow of waste water to be screened and the rods of the second row are staggered relative to the rods of the first row, laterally of the direction of flow.
Double screens and double-rack grates having properly aligned rods and rows of rods in staggered arrangement are known. Extra-fine screens or grates refer to screens or grates with a clearance between adjacent rods of less than 10 mm, preferably less than 8 mm. This type of extra-fine screen or grate is reliable in operation for the retention of granular or similar matter. However, when used with waste water and sewage flows carrying fibrous substances, serious failures will result due to clogging deposits of fibers encircling the surfaces of the rods. Once such clogging and flow obstruction have taken place by accumulation, these undesired surface coverings keep increasing during operation and in the end can no longer be removed by appropriate cleaning agents. The breakdown or failure of such a screen or grate due to clogging finally can be overcome only by exchanging it with a new screen or grate and scrapping the old one.
Obviously, complicated processes of flow between the rods play an important role in the formation of such cloggings that accumulate on the surfaces of the rods. It has been found that, even though staggered rectangular sections enable the setting of very narrow clearance widths, such arrangements are extremely vulnerable to the dirt conditions mentioned above.